Applying coatings or adhesives to various widths of sheet materials moving at high speeds has been a perennial problem in the paper industry. The effective face width of some types of coaters can be controlled with dams or similar means. In other types, size presses for example, the coating is applied to the full width of the rolls and any excess beyond the edge of the sheet is flung off or flows off of the rolls. Adhesive applicators for corrugating machines are similar to size presses in that the adhesive material covers the full width of the applicator roll. Transfer of adhesive to the edges of the lower corrugating roll is prevented by allowing the corrugated medium to balloon out slightly by centrifugal force as it passes the applicator. This causes other problems, however, such as nonuniform flutes. One approach to this problem is shown in Bruker, Canadian Pat. No. 548,505, which uses a variable width adhesive supply to the applicator roll.
New technology has recently been developed using nonaqueous adhesives, usually hot melt types, which give greater versatility in corrugated board manufacture. U.S. Pat. No. 3,518,142 to Dooley is an example. The availability of these adhesives has sparked other improvements in corrugator design, as shown in the Wolvin and Morris U.S. Pat. No. 3,972,763. This system not only captures the advantage of nonaqueous adhesives but it also is designed to overcome problems of nonuniform flutes. Wolvin et al have utilized adhesive spreaders similar to those shown in Beck et al, U.S. Pat. No. 3,688,736 and Brandenberg, U.S. Pat. No. 3,789,795. All three systems apply the adhesive as a series of parallel stripes or bands. This is advantageous for spread control with nonaqueous adhesives and particularly with hot melt systems because of their much higher viscosities than conventional starch-based adhesives. Conventional spreaders using hot melts, as shown in the aforementioned Dooley patent, have not been successful because of over-spreading of these highly viscous materials. This is not tolerable for several reasons, not the least of which is the much higher cost of the hot melts. Direct extrusion of adhesive onto the flute tips, as shown in FIG. 6 of the Dooley patent, has similarly failed to give a product of acceptable quality and price.
While the first generation adhesive spreaders of the Wolvin et al patent have worked successfully, they have not been totally without problems. These relate both to uniformity of spread and to edge control, particularly at the single facer.
A second generation adhesive spreader is shown in pending application, Ser. No. 836,181, filed Sept. 23, 1977, assigned to the present assignee, which is herein incorporated by reference. Instead of pickup rolls operating in a tray of adhesive with spread being controlled by doctoring devices, this shows the use of a uniformily apertured manifold flowing adhesive onto a transfer roll. Adhesive not transferred to the tips of the flutes of the corrugated material is doctored off of the roll into a tray. From that point it is reheated, filtered and recycled through the applicator. Spread control is achieved by a variable rate pump that supplies adhesive to the manifold.
This manifold type applicator has been a significant improvement over the original adhesive applicators but it too has left some important problems unaddressed. One of these is the perennial problem of edge control. Another has been unique to the manifold system. This is a problem of occasional orifice plugging due to high-temperature-induced chemical changes in the adhesive. One proposed explanation has been that there are eddys adjacent to the manifold walls in the areas between the orifices. Adhesive viscosity may build up in these low- or no-flow areas until gels form. Periodically these may break loose and are swept into an orifice whereupon plugging occurs. The exact cause is not known for certain, however.
In an attempt to gain edge or width of spread control the use of pistons in the ends of the manifold was tried. These could be varied axially along the manifold to control the number of orifices in communication with the adhesive source. This was successful for its intended purpose but it too created a new and intolerable problem. When the effective manifold width was adjusted from a relatively wider to a relatively narrower width, the adhesive would firmly solidify in the now unused apertures at the extreme ends of the manifold. In the warm environment this adhesive would apparently oxidize to form hard plugs which frequently had to be removed mechanically when it was again desired to bond a wider sheet.